This fixes the compatibility problem with Firefox 40.0. Still it does not work, because of https://bugzilla.mozilla.org/show_bug.cgi?id=1193088
in Firefox: Selecting applications->Portable document format (PDF) -> Use Adobe Acrobat (in Firefox) needs restart before becoming effective
But without this one line fix, the addon just does not load due to an unresolved import. If the but in Firefox is fixed, the addon should work again.